Sintered quartz glass products and methods for making same

ABSTRACT

A number of unique processes are disclosed for manufacture of sintered high-purity quartz glass products in which a shaped silica body or preform is made from an aqueous slurry of micronized silica particles by gel casting, slip casting or electrophoretic deposition. The silica particles may comprise a major portion by weight of crystalline silica. In one embodiment of the invention the sintered quartz glass is transparent, substantially bubble-free and suitable for scientific or optical uses. In another embodiment the porous silica preform is fired in steam to increase the hydroxyl content and then nitrided in a nitrogen-hydrogen reducing atmosphere. A minute amount of chemically-combined nitrogen in the high-purity quartz glass is sufficient to provide a tremendous improvement in physical properties and an incredible increase in the resistance to devitrification.

This application is a continuation of application Ser. No. 08/804,234,filed Feb. 22, 1997, now U.S. Pat. No. 6,012,304 which is a continuationof application Ser. No. 08/269,002, filed Jun. 30, 1994 now abandoned,which is a continuation-in-part of application Ser. No. 07/767,691,filed Sep. 30, 1991 now U.S. Pat. No. 5,389,582.

The present application relates to the manufacture of quartz glassproducts having new or improved properties and to novel processes formaking such products from porous silica preforms. One preferredembodiment of the invention involves nitrided vitreous quartz productswith outstanding physical properties. Other embodiments involve the useof silicon alkoxides, such as ethyl silicate (TEOS), for impregnation ofporous silica preforms prior to nitriding or sintering of the preformsand for gel-casting of quartz glass products. Another embodimentinvolves the making of quartz glass articles by electrophoreticdeposition. Another involves the production of such glass articles byslip casting in special silica molds.

BACKGROUND OF THE INVENTION

It has been known for many years that nitrides of silicon haveproperties different from silicon dioxide and that some of theseproperties might be advantageous in certain applications. Siliconnitride and silicon oxynitrides can be produced in various ways as byreaction of silicon and/or silicon dioxide with ammonia, and products ofthis type would have utility for some special applications.

However, there are many reasons why the commercial use of such productshas been very limited, why research relating to nitrided siliconproducts has not been extensive, and why large capital investment forresearch and development in this area did not appear to be justified. Itis difficult and expensive to produce silicon nitride products orsilicon oxynitride products. Silicon dioxide (silica) does not reactreadily with nitrogen, although it is possible with appropriate reactionconditions to produce oxynitrides by reacting particles of silica withanhydrous ammonia.

In the field of microelectronics, scientists have given someconsideration to possible uses of silicon oxynitride films because ofthe unique dielectric properties and other properties. Such films can beproduced by chemical vapor deposition or by nitridation of siliconsurfaces or thin silicon-dioxide films. Thin silica films made by asol-gel process can be penetrated by ammonia, perhaps because of themicroporosity and cracking of the dried film. At a temperature of 1000°C. to 1200° C., anhydrous ammonia can react with the silica film toproduce oxynitrides with special properties.

Consideration has also been given to the manufacture of glass orglass-ceramic products from compositions containing silica (SiO₂) andnitrogen (N) as base components as described in Corning Patent No.4,222,760. However, that patent points out that the practicalglass-forming region is quite small in the simple ternary SiO₂—Al₂O₃—Nsystem (FIG. 8) and is essentially non-existent in the simple binarySiO₂—N system.

Silicon oxynitride glasses can be produced by melting a mixture of oxideand nitride powders at a high temperature, such as 1600° C. to 1700° C.or more. Oxides of aluminum and other metals may be used (i.e., Ca, Li,Mg or Y). The nitrogen source may be Si₃N₄ or AlN, for example. Theoxynitride glass is potentially useful in making special plate glass orglass fibers (See U.S. Pat. No. 4,609,631).

Oxynitrides have some desirable properties which may be superior tothose of quartz glass and may have potential value in the semiconductorindustry. However, it appears that such potential, if any, has yet to berealized and that the use of oxynitride glass in connection with thecommercial manufacture and processing of silicon-wafers and othersemiconductor devices has not been found worthwhile.

To date there has been no practical substitute for quartz glass in thecommercial manufacture of silicon semiconductors. The modern glasscrucibles used in Czochralski (Cz) crystal-growing furnaces have beenformed of silica having a very high purity (i.e., a purity of at least99.99 percent). Substantial amounts of nitrogen cannot be tolerated inCz crucibles. For more than two decades the manufacturers of siliconcrystal have insisted that the crucibles used in crystal-growingfurnaces be transparent and free of significant amounts of nitrogen orcristobalite.

Because of the importance of microelectronics and computers, there is ahigh demand for ultra-pure silica glass in the manufacture of modernmicro-chips. The semiconductor industry is becoming increasinglyintolerant with respect to contaminants in quartz glass. In order tomeet modern requirements for the processing of semiconductor wafers, aglass should contain at least 99.995 percent by weight of silica. Theultra-pure synthetic fused quartz commonly used for this purpose usuallyhas a purity of about 99.999 percent.

Prior to the present invention, the presence of significant amounts ofchemically-bound nitrogen in a quartz glass used in semiconductormanufacture would have been considered highly undesirable. Nitrogenheretofore appeared to be an impurity to be avoided.

The percentage of the nitrogen impurity in a commercial quartz glass islow but is not often measured or reported because of the difficulty ofascertaining the nitrogen content with reasonable accuracy. Theanalytical detection problem is another good reason why the unusualproperties and advantages of chemically-bound nitrogen were heretoforenot understood nor appreciated in the glass industry.

For several decades vitreous silica products essentially free ofcrystalline silica have been used extensively because of exceptionalthermal shock resistance and other advantageous physical properties.However, these products have a limited useful life when heated above1200° C. and other disadvantages because of limited resistance todeformation, the devitrification of the glass, and the damage resultingfrom the crystallographic alpha-beta inversion during heating andcooling of the devitrified glass. There has been a need for a practicalsolution to these problems for several decades, particularly thedevitrification problem, but no simple solution was found prior to thepresent invention.

There has also been a need to remedy other deficiencies in certainproducts and processes involving the use of quartz glass or vitreoussilica. For example, serious problems have been encountered whenattempting to cast elemental silicon in silica molds, making itnecessary to tolerate the expense and inefficiency of temporarybreakaway casting molds.

In the semiconductor industry, modern epitaxy reactors, diffusionfurnaces, CVD equipment and other high-temperature equipment have agreat need for effective thermal radiation heat shields. There have beensome attempts to meet this need, but they have been crude and generallyunsatisfactory.

Gel Casting

There has also been a need for better methods for molding high-purityquartz glass products including improvements in conventional slipcasting methods. Because of the limitations of slip casting, otherpractical casting processes and techniques are sorely needed, such aselectrophoretic deposition and gel-casting, (See U.S. Pat. Nos.4,092,231 and 4,622,056), but this need has not been satisfied. Prior tothe present invention, there was no practicable and commercially viablemethod for gel casting a variety of shaped quartz glass products.

In the field of glass and ceramics, ethyl silicate has been used formore than 50 years as a binder (See U.S. Pat. No. 1,909,008). The use ofalkyl silicate binders is a feature of the well-known “Shaw Process”developed by Avnet-Shaw Corporation and disclosed, for example, in U.S.Pat. Nos. 2,795,022 and 3,172,176. Ethyl silicate binders are commonlyused in the manufacture of various silica products. Ethyl silicate canbe added to a slurry in small amounts and hydrolyzed to serve as abinder and can be used in various injection, extrusion and pressingtechniques as described in U.S. Pat. No. 3,423,216 and U.S. Pat. No.4,789,389, for example. Although the properties and uses of ethylsilicate have been well known for decades, the full potential of thismaterial in the manufacture of glass has not been realized norappreciated.

It has been known for several years that ultra-pure synthetic quartzglass can be produced by sol-gel techniques or by the hydrolysis of asilicon chloride or a silicon alkoxide (See U.S. Pat. No. 4,572,729).Optical glass is commonly made by flame hydrolysis of silicontetrachloride (See Corning Patent No. 3,806,570). The typical syntheticfused quartz has a purity of 99.995 percent or greater.

Silicon alkoxides, such as tetraethyl-orthosilicate (known as TEOS orethyl silicate), can be employed in a sol-gel process for manufacture ofultra-pure quartz glass as disclosed in U.S. Pat. Nos. 4,572,729 and4,789,389. In the latter, a gel formed from hydrolyzed TEOS is dried toproduce small granules which are then sintered to produce ultra-puresynthetic fused quartz. The ultra-pure quartz granules made in this wayare then wet milled to form a slurry and used to form a shaped porousgreen body or preform by slip casting, injection molding or othercasting methods. According to the patent the green body may be sinteredto full density in a vacuum furnace and thereafter placed in a hippingfurnace (e.g., See U.S. Pat. No. 4,349,333). The hot isostatic pressingtypically involves a pressure of at least 1,000 psi to eliminate gasbubbles in the glass.

Heretofore, various sol-gel processes have been proposed for casting ofshaped silica bodies. They involve casting a “green” silica body, dryingthe body, and firing or sintering the body to form a dense silicaproduct. For example, in U.S. Pat. Nos. 4,680,046 and 4,680,048, asol-gel casting process is disclosed in which ultrafine particle silica,such as fumed silica, is added to a hydrolyzed ethyl silicate to enhancethe yield and to reduce the porosity of the dry gel (See U.S. Pat. No.4,680,046). The ultrafine silica particles tend to cause the glass tofoam, but such tendency is reduced by selecting the proper amount to beadded (See U.S. Pat. No. 4,680,045). The ultrafine silica may be acommercial fumed silica, such as Aerosil or Cab-O-Sil, or a comparablesilica made in a different manner, as by flame hydrolysis of silicontetrachloride (See U.S. Pat. No. 4,801,318).

Unfortunately the sol-gel casting processes heretofore proposed havebeen disappointing and have suffered from serious problems, particularlythe cracking and shrinkage problems described in said U.S. Pat. No.4,789,389. Such processes have not met with commercial acceptance orsuccess and have not provided a competent, reliable and practical methodfor casting shaped quartz glass products. They have failed to satisfythe important needs of the industry.

For these reasons, as pointed out in said U.S. Pat. No. 4,789,389,sol-gel techniques have been employed for manufacture of ultrapuresynthetic quartz granules rather than for commercial gel casting ofquartz glass products.

Electrophoretic Deposition

Electrophoresis has been known for more than 100 years and has been usedin a variety of ways as a technique for the coating of metal articles.This technique has been employed for depositing metals, oxides,phosphors, rubber, paints, polymers and other materials using bothaqueous and non-aqueous media. It has been used extensively in thecommercial manufacture of rubber products from latex and in automotivepainting.

In the ceramic manufacturing industry, the use of electrophoreticdeposition has been rather limited. There are a few processes which havehad substantial commercial value. One involves the electroforming ofbeta-alumina articles for use in high-energy sodium-sulfur andsodium-halogen batteries (see U.S. Pat. No. 3,946,751) and anotherinvolves the formation of thin continuous strips of clay, suitable forcutting into tiles or plates (see Chronberg U.S. Pat. Nos. 4,092,231 and4,170,542). So far the use of the Chronberg process for manufacture ofceramic tile from clay suspensions has not been fully exploited.

Electrophoretic deposition is well suited to the manufacture ofbeta-alumina articles by the process described in U.S. Pat. Nos.4,073,711 and 4,279,725 (General Electric). In that process an organicsuspension of beta-alumina particles is employed using amyl alcohol asthe liquid media because of its dielectric properties. The particles aredeposited on an electrically-charged mandrel to form thin-walled tubeswith a diameter of about 1 centimeter and a wall thickness of about 1millimeter as described in U.S. Pat. No. 4,279,725.

Electrophoresis has also been proposed as a method for speeding up theslip casting of clay earthenware or pottery as disclosed in U.S. Pat.Nos. 3,718,564 and 4,121,987. It has also been proposed for themanufacture of porcelain articles and porcelain-coated articles (seeU.S. Pat. Nos. 3,484,357; 3,575,838 and 4,708,781).

The electrophoretic processes described above for use in forming ofceramic articles commonly employ aqueous suspensions containingadditives, such as polyacrylic acid, triethylamine, ethanol, sodiumcarbonate, sodium hydroxide, sodium silicate, surface-active agents,deflocculants, etc.

In general, organic liquids are considered superior to water as asuspension medium for electrophoretic forming. The use of water-basedsuspensions causes a number of problems including gas evolution at theelectrodes. This can cause, bubbles to be trapped within the deposit.Special means have been proposed to minimize this bubble problem as byusing a porous membrane and depositing the particles on the membrane asdisclosed in U.S. Pat. Nos. 4,684,386 and 4,689,066. The bubble problemis less serious when using an organic suspension instead of an aqueoussuspension.

The latter patents (U.S. Phillips Corporation) relate to the manufactureof thin-walled quartz-glass tubes for optical waveguides. U.S. Pat. No.4,689,066 describes manufacture of a transparent glass tube with adiameter of 19 mm and a wall thickness of 1.2 mm from an homogenizedanhydrous suspension of colloidal silica containing a quaternaryammonium compound. The organic media may be ethanol. The silicaparticles typically have a particle size of 15 to 100 nanometers (0.015to 0.1 microns) with an average particle diameter of about 40nanometers.

Electrophoretic deposition of coatings and the formation of thin-walledarticles from colloidal silica can be feasible if the deposit isrelatively thin. However, the deposited coating loses its conductance asthe thickness of the deposit increases, thus retarding the rate ofdeposition. Because of this self-limiting characteristic, the buildup inthe electrical resistance of the deposit can be a major problem whenattempting to produce articles with substantial wall thickness.

There are a number of reasons why electrophoretic forming processes haveso far achieved little commercial success. There are seriousshortcomings in the fundamental understanding of the subject, and it isdifficult to predict whether a given suspension will depositelectrophoretically in the desired manner. Laboratory testing hasindicated that a large number of different powders can be depositedincluding barium and calcium carbonates, alumina, magnesia, zinc oxide,silica, titanium dioxide, indium oxide, tungsten carbide and variousmetals and phosphors.

It would be desirable to be able to predict from suitable parameterswhether an electrophoretic deposition process will produce the desiredresults. The most commonly used parameters are zeta potential andelectrophoretic mobility, but zeta potentials are difficult to measureor to interpret. Unfortunately there is no satisfactory theory thatcovers and explains all observations on electrophoretic deposition, andthe subject is not well understood. Theoretical mathematical analysishas been attempted but is questionable because the equations used arebased on assumptions regarding particle size and shape and theoreticalmodels of doubtful validity (e.g., conveniently assuming that thecharged particles are spherical when that is not true).

It appears that, because of lack of adequate information,misconceptions, prior failures, lack of experience or other reasons, theversatility of the advantages and potential advantages ofelectrophoretic deposition in the manufacture of improved glass andceramic products were heretofore not appreciated prior to the presentinvention. In any event, research and development work in the field ofelectrophoretic deposition has been neglected, and the ceramic industryhas relied on other forming processes.

In the field of investment casting where refractory shell molds areformed by the usual “lost-wax” process, it has been suggested thatelectrophoresis be employed during manufacture of the shell molds asdisclosed in Szabo U.S. Pat. Nos. 3,8509,733 and 3,882,010. In theproposed Szabo process the wax patterns are coated with graphite anddipped in an electrically-conductive coating suspension. The Szabopatents recognize that gas evolution at the anode or depositoryelectrode creates major problems and that it is difficult to providereliable results by electrophoretic deposition. These patents do notprovide a reliable and commercially satisfactory process of substantialimportance.

The problems associated with electrophoretic deposition are discussed inNorton U.S. Pat. No. 4,357,222 including the major problem of gasformation at the depository electrode (anode) from electrolysis of theslip liquid which causes serious flaws in the cast part. The Nortonpatent minimizes this bubble problem by providing a specialnon-conducting rubber mold having a relatively small anode at the bottomof the mold which forms only a small fraction of the forming surface ofthe mold and by moving one electrode relative to the other. A sphericalcasting of substantial size can be molded by filling the mold cavitywith a suitable casting slip such as a suspension composed of about 86percent by weight of silicon carbide, about 14 percent by weight ofwater and 0.1 percent by weight of sodium silicate. If the mold cavityis filled with a slip composed of about 50 percent water, about 50percent elemental silicon and about 0.5 percent sodium silicate, asilicon casting is produced which can be converted to silicon nitride bystandard nitriding.

The stability of the slip is less important in the non-conducting rubbermold of the Norton patent because the depository anode is at the bottomof the mold and attracts the particles in the same direction as gravity.

Unfortunately the Norton process has very limited utility and isunsuitable for formation of thick-walled articles, such as tanks,crucibles or other receptacles, where gravitational force can causeserious adverse effects.

For many years there has been a need for a simple, versatile andefficient process for commercial manufacture of relatively thick quartzglass products from fine silica particles. The electrophoreticdeposition processes described in the prior art discussed previouslyfail to meet this need.

Slip Casting

Slip casting is today the most practical process for forming quartzglass articles from fine silica particles. For at least several decades,conventional slip casting in plaster of Paris molds has been used in thesemiconductor industry to make transparent Cz crucibles and other quartzglass receptacles of the highest quality with a purity from 99.95 to99.99 percent or greater. Such crucibles are also produced commerciallyin rotating molds by arc-fusion methods which can be advantageous whenproducing crucibles or receptacles with a substantial wall thickness inexcess of 0.3 inch. However, arc-fusion cannot be used to make trays orreceptacles having flat bottom or side walls and non-circular shape asare commonly produced by slip casting. Almost all of the silicacrucibles used by the semiconductor industry in modern Cz furnaces forsilicon crystal manufacture are high-purity quartz glass cruciblesproduced either by arc-fusion methods or by conventional slip casting inplaster molds.

The standard plaster of Paris molds used in slip casting are a marveland have enjoyed unparalleled success for many decades. Such molds havea unique and incredible open-cell structure which is almost ideal forslip casting and which is unmatched by any other material proposed for acomparable use. The conventional plaster molds provide a uniquecombination of high porosity and uniform small pore size and enjoy theexcellent capillary action needed for practical aqueous slip casting.

Other inorganic and organic materials have been proposed for use in slipcasting molds, such as graphite and synthetic polymers (i.e., plastics),but they are inferior and do not provide comparable capillary action.Such proposals do not involve an improvement with substantialsignificance or importance in the commercial manufacture of high-purityquartz glass products for the semi-conductor industry.

While conventional plaster of Paris molds have satisfied the needs ofthe semiconductor industry with respect to the important quartz glassproducts, they have obvious limitations. They are generally unsuitablefor slip casting at a pH below 7 and usually function best when usingaqueous slips with a pH of 7.5 or so. With such plaster molds it ispossible to produce quartz glass receptacles and transparent Czcrucibles of the highest quality with an ultrapure inner surface, butslip casting in such plaster molds can be inefficient, uneconomical, orunsatisfactory if the receptacle being formed has a relatively thickwall. It may require 6 to 8 hours or more to slip cast a silica preformwith a wall thickness of one-half inch or so. If the wall thickness ofthe desired silica product is doubled, it may take more than four timesas long to perform the slip-casting operation.

This problem has been recognized for several decades and attempts havebeen made to speed up the slip-casting operation, but a practical andcommercially satisfactory solution to the problem was not found.Heretofore, attempts to find a satisfactory substitute for plaster ofParis molds in the slip casting of silica glass products have been foundwanting.

Conventional plaster molds have other limitations and disadvantageswhich are inherent and perhaps cannot be eliminated. The strength anduseful life of a plaster mold is limited. The mold can be seriouslydamaged if it is subjected to high temperature or dried at a rapid rate.Plaster of Paris molds should be dried at a moderate temperature ofabout 45° to 50° C. A typical drying operation is necessarily very slowand usually requires at least two days.

Helium-sintered Quartz Glass

During the last two decades sintered transparent quartz glass articles,such as Cz crucibles, acid tanks and other receptacles, have beenmanufactured by a process of the type disclosed in U.S. Pat. No.4,072,489 wherein a porous silica body or preform formed by slip castingis dried and fired and later sintered rapidly in a helium atmosphere tofull density on a perforated shaping mandrel in an induction furnace.Semiautomatic equipment similar to that shown in said patent wasemployed with cycle times of 6 to 15 minutes depending on the size ofthe glass article. The short cycle minimized the amount ofdevitrification during sintering, and the sintering temperature wasraised above 1720° C. and above the melting point of cristobalite toproduce a full density transparent glass.

The above process was used commercially to avoid excessive manufacturingcosts and provided admirable results, but the transparent quartz glasscontained substantial amounts of undesirable voids or gas bubbles andusually had unsatisfactory optical properties. The causes of the largerbubbles were not fully understood and a satisfactory solution to theproblem was not found prior to the present invention. The transparentglass products commonly contained significant numbers of visible bubbleswith diameters from 150 to 400 microns or more and were unacceptable tomany customers for this reason.

For many years there has been a great need for a reliable and economicalcommercial process to produce transparent helium-sintered quartz glassproducts with a minimal bubble content and a minimal number of visiblebubbles.

SUMMARY OF THE INVENTION

One preferred embodiment of the present invention relates to thenitriding or nitridation of porous silica preforms and involves newtechnology which appears to be a giant step forward and a breakthroughof potentially great importance in the field of nitrogen-containingsilica or silicon oxynitrides. Incredible improvement in the physicalproperties of a high-purity quartz glass can be obtained byincorporating a minute amount of chemically-bonded nitrogen in thesilica.

The present invention also involves the use of silica sols, particularlythose made by hydrolyzing silicon alkoxides, such as ethyl silicate(TEOS). One preferred embodiment relating to the gel casting ofhigh-purity silica glass is very important because of the great need forsuch a casting process as an alternative to slip casting and theelimination of the calcium contamination problems associated withconventional slip casting. The unique gel-casting process happens to bewell suited to the production of nitrided quartz glass according to thefirst-named embodiment, or ultra-pure transparent bubble-free quartzglass according to another embodiment of the invention. It can also beused to make special high-porosity silica molds remarkably well suitedfor use in slip casting.

Another embodiment relates to a unique electrophoretic depositionprocess particularly advantageous in the commercial manufacture ofquartz glass products with walls of substantial thickness.

Nitrided Quartz Glass

Said first embodiment is remarkable not only because of the difficultyin forming substantial amounts of chemically-bound nitrogen but alsobecause of the difficulty in measuring or detecting the amounts beingformed or in ascertaining any benefits therefrom. The improvementobtained in the resistance of quartz glass to devitrification was quiteunexpected.

In accordance with this invention various measures are taken to promotenitridation of silica including the use of ammonia, the incorporation ofcatalysts, such as calcium, the use of high pressures, and thepretreatment of the silica to provide surface reactive groups, such ashydroxyl or halogen groups. Carbon monoxide is also employed toadvantage. However, it is difficult to incorporate more than a verysmall amount of such reactive groups in a porous silica preform of thetype used to form typical quartz glass products and difficult toincorporate substantial amounts of chemically-bound nitrogen in a quartzglass. It is also impractical to employ extremely expensive equipment toprovide the high pressures which are desirable for effective nitriding.

The present invention does not require such expensive equipment. In oneembodiment pressure is created within closed pores (or bubbles) of theglass by the surface tension of the glass surrounding the pores.

For example, in carrying out the invention of said first embodiment, asilica preform formed from fused quartz particles by slip casting orother suitable method and having a porosity of 10 to 40 volume percentcan be dried, hydroxylated by firing in air containing steam, and thennitrided in anhydrous ammonia at a high temperature, such as 1000° C. to1200° C. The nitrided preform can then be presintered for 1 to 3 hoursor more at a temperature of 1400° C. to 1500° C. to increase the densityabove 90 percent and to close the pores before the final sintering to atemperature above 1700° C. The chemical bonds between the nitrogen atomsand the silicon atoms tend to be unstable at temperatures above 1500°C., and pressure is created by the nitrogen as a result of thatinstability. The opposing pressure needed to maintain stability resultsfrom the surface tension of the glass as further explained hereinafter.

Gel Casting

The second important embodiment of the present invention relates to agel-casting process for making quartz glass products, the basic featuresof which are disclosed in copending application Ser. No. 07/767691,filed Sep. 30, 1991. Such disclosure is incorporated herein by referenceand made a part of this application.

Said copending application refers to the gel-casting process of theinventor, Ted A. Loxley, which involves in situ hydrolysis of a siliconalkoxide (e.g., ethyl silicate) in a slurry or slip containingmicronized particles of vitreous silica. The slurry has a high solidscontent of at least 80 percent by weight before the ethyl silicate isadded.

In carrying out the invention of this second embodiment, a conventionalslurry is preferably prepared in a ball mill as typically used for slipcasting of quartz glass. The water in the slurry (e.g., from about 15 toabout 20 percent by weight) is employed for in situ hydrolysis of theethyl silicate (TEOS) which is added to the slurry. The mole ratio ofwater to ethyl silicate is at least about 2:1 and is usually from 3:1 to6:1. An acid, such as hydrochloric acid, formic acid or nitric acid, maybe employed to promote the hydrolysis during extended mixing.

Before the slurry containing the hydrolyzed ethyl silicate is poured orfed into a mold, the pH is adjusted by adding a weak base, such asmorpholine or urea, to initiate gelling. After gelling, the casting canbe dried and fired to produce a porous silica body or preform with aporosity of from 15 to 40 volume percent. Such a preform is well suitedfor manufacture of nitrided quartz glass using the various techniquesdescribed above with respect to said first-named embodiment of theinvention. Such a porous body or preform with a porosity of about 25 to30 volume percent and a network of open pores of minute size (e.g., fromabout 1 to 4 microns) is remarkably well suited for use in slip casting.

Helium-sintered Quartz Glass

Other embodiments of the present invention relate to the manufacture oftransparent quartz glass products with a reduced bubble content andparticularly helium-sintered products with a minimal bubble content orwith sound optical properties.

A third preferred embodiment of the present invention involves theimpregnation of a porous silica preform formed by slip casting,isostatic pressing, electrophoretic deposition or gel casting, forexample. After drying and firing, the preform is soaked in or thoroughlyimpregnated with a suitable silica sol (e.g., a hydrolyzed siliconcompound, such as TEOS), and then gelled, dried and fired before asubsequent nitriding treatment or a final sintering in an inductionfurnace.

It has been discovered that such impregnation of the porous preform witha hydrolyzed silicon alkoxide (e.g., ethyl silicate) provides importantadvantages in the commercial manufacture of transparent quartz glassreceptacles, such as crucibles, bell jars and acid tanks. For somereason such treatment of the preform improves the purity of the productby helping to remove sodium ions and other metal ions. It alsofacilitates the sintering operation and makes it possible to reduce theformation of gas bubbles in the glass during the final helium sinteringoperation.

The larger pores of a slip-cast silica preform are more apt to causesignificant gas bubbles in the sintered glass. These large pores soak upthe hydrolyzed alkyl silicate and permit gelling thereof inside thepores. The result of the alkyl silicate impregnation seems to be asmaller and more uniform pore size, better suited to the production oftransparent quartz glass which has a minimal bubble content.

Other means may be employed to reduce the bubble content of the quartzglass. It has been found that the average bubble diameter can be greatlyreduced and that the number of visible voids or bubbles can bedrastically reduced by avoiding the use of plaster of Paris molds. Theseadvantages are obtained, for example, when the plaster molds used forslip casting are replaced by silica molds as described hereinafter.

Other modifications in the production process can be helpful inminimizing the bubble problem including changes in the slurry and in thesintering procedures. It can be advantageous to sinter the fired poroussilica preform in two stages over a substantial period of time ratherthan in a single furnace in 6 to 12 minutes as in U.S. Pat. No.4,072,489. Rapid sintering to a temperature of 1600° C. to 1750° C.according to said patent is non-uniform and intended to provide atemperature gradient. In a two-stage process wherein the preform isfirst sintered for 30 minutes to 3 hours or more at a temperature offrom about 1400° C. to about 1500° C., the first-stage of the sinteringcauses more uniform heating of the silica and promotes the formation ofcells with a more uniform pore size.

A two-stage process is particularly effective when the porous preform ismounted on a porous or perforated mandrel and flushed with helium for 20to 30 minutes or more during the first stage sintering. It has beendiscovered that such pre-sintering in helium at a temperature of 1450°to 1600° C. followed by the standard helium sintering for 5 minutes ormore to a temperature of 1750° C. can produce superior transparentquartz glass with a minimal bubble content.

The two-stage sintering process can be carried out in such manner as tocause a substantial increase in the density of the preform and/or toseal the pores thereof during the first stage. The first-stage sinteringcan be carried out in an atmosphere of helium or in a vacuum at asuitable low pressure (preferably below 5 torrs). If the density of thepreform is increased to 95 volume percent or so to close and seal thepores, then the second-stage sintering in the induction furnace can becarried out in an atmosphere of argon or other inert gas.

Nitrided Glass

In carrying out the invention of the first-named embodiment, a shapedsilica body or preform with a porosity of 10 to 40 volume percent isformed from a refractory silica composition or a slurry of fine silicaparticles by slip casting, gel casting, electrophoretic deposition,isostatic pressing, injection molding or other suitable method (see U.S.Pat. Nos. 3,222,435 and 3,619,440). The porous silica preform is formedand treated in such a manner that, after drying and firing, it containsa substantial amount of chemically-bound hydroxyl groups and/or othersuitable surface reactive groups (e.g., at or near the inner surfaces ofthe pores) which promote nitridation of the silica. These reactivegroups are usually hydroxyl or silanol groups rather than halogengroups. The fired porous silica preform is then nitrided in a nitrogenreducing atmosphere (e.g., an atmosphere of anhydrous ammonia maintainedat a suitable high temperature from 850° C. to 1200° C.). To assure thatthe pores of the preform are filled by the ammonia gas or a mixture ofnitrogen gas and hydrogen gas, a substantial vacuum can be employed toremove air or other gas from the pores of the preform before the ammoniaor other nitrogen-containing reducing gas is introduced to those pores.Also a pressure differential can be provided to force the ammonia ornitrogen gas through the porous preform.

The final sintering of the preform to a high density, such as 98 to 99weight percent, can be carried out in an electric induction furnacegenerally as disclosed in U.S. Pat. No. 4,072,489 using a nitrogenatmosphere rather than an atmosphere of helium. The glass is usuallyheated to at least 1700° C. during sintering and is preferably heated toabout 1750° C. or above the melting point of cristobalite to eliminatecrystalline silica.

The porous silica preform can be treated prior to nitridation to obtainimproved results. The treatment can include impregnation with ahydrolyzed silicon alkoxide as described hereinafter and can include ahydroxylation treatment to increase the number of hydroxyl groups,silanol groups or other reactive groups which promote nitridation (e.g.,chemical bonding of nitrogen to silicon atoms). In a preferredembodiment of the invention, the porous preform is heated in a furnaceatmosphere of air or oxygen and steam to a high temperature, such as400° C. to 1100° C., to effect a substantial increase in the hydroxylcontent of the glass prior to the nitriding step.

Optionally the porous silica preform can be impregnated with ahydrolyzed silicon alkoxide, such as TEOS, dried, and fired before theabove-described steam treatment or the nitriding operation.

A number of unique and remarkable products can be produced whenpracticing the present invention. Nitrided quartz glass products madeaccording to the invention exhibit remarkable physical properties andcan be of great commercial value. The nitridation of a porous vitreoussilica preform in accordance with the invention apparently causesnitrogen atoms or amine groups to become chemically bonded to surfacesilicon atoms of the vitreous silica, thereby effecting a remarkablechange in the physical properties of the quartz glass even when thenitrogen content is barely measurable (e.g., below 0.005 percent byweight).

The resistance of the quartz glass to devitrification at hightemperatures (e.g., 1100° C. to 1300° C. or higher) and the useful lifeof the glass under harsh conditions can be drastically improved bynitridation, perhaps more than fifty-fold and possibly two orders ofmagnitude. At the same time the high-temperature viscosity or resistanceof the glass to deformation at high temperatures, such as 1400° C. orhigher, can be increased dramatically. Because of their remarkableproperties, nitrided quartz glass products made according to theinvention are valuable for a wide variety of uses in the chemical andelectronic arts and other scientific arts and include bell jars,crucibles, tanks, trays, and plates and tiles for furnaces, reactors andhot-wall applications. Such products are particularly useful in thesemi-conductor industry and the field of microelectronics because ofextreme purity, uniformity and reliability.

The nitrided quartz glass of this invention with a density from 98 to99.5 percent by weight or more and a silica content of at least about99.99 percent by weight is remarkably well suited for some specialapplications. The nitrided glass seems to have unique surfacecharacteristics not possessed by conventional quartz glass. It has beenfound that the nitrided glass, unlike conventional quartz glass, issuited for use as a permanent shaping mold for casting moltenhigh-purity silicon, thereby eliminating the need for disposablebreakaway molds which were heretofore used in molding silicon ingots.

The opaque nitrided quartz glass of this invention is particularlyvaluable for radiation heat shields used in CVD furnaces for chemicalvapor deposition and in epitaxy reactors, diffusion furnaces and otherfurnaces used in the semiconductor industry. Such glass is extremelywell suited for such uses. In epitaxy reactors, for example, the newhigh-density heat shields of this invention are so superior to thosepreviously used that the old shields are considered impractical.

The heat shields of the present invention are remarkable in many ways.They normally have a high density from 98 to 99 percent by weight,usually at least 98.4 percent. This minimizes the contamination problem.This invention involves the discovery that small pores of minute ormicron size provide optimum resistance to radiation. When the pores havea small diameter, such as 1 to 3 microns, and a large number of poreshave a diameter which is close to the wave length of the radiation, theefficiency of the heat shield can be very high.

This invention is remarkable in that it makes possible formation of analmost ideal network of small cells. This is made possible by providingamine groups or nitrogen atoms which are apparently bonded to siliconatoms and which are unstable at the higher sintering temperatures so asto create a vapor pressure adequate to resist the compressive forces dueto the surface tension of the glass. The result is white, opaque glassof high density with pores having a minute size (e.g., 1-4 microns). Ifthe density is increased from 98.5 percent to full density in a hippingfurnace, the glass can become transparent.

Electrophoretic Deposition

Another embodiment of the present invention relates to the formation ofporous silica preforms by a unique electrophoretic deposition processwherein a mold with an electrically conductive shaping surface isimmersed in an aqueous suspension or slip substantially free of ionicimpurities and containing electrically-charged micronized particles ofhigh-purity silica. At least a portion of the slip is preferablyagitated to provide more uniformity during the deposition process usingsuitable means, such as an ultrasonic transducer or a rotary mixer, tomaintain the silica particles in suspension.

The particle size of the high-purity fused quartz is kept withinpredetermined ranges so that the deposits on the metal molding surface(anode) are highly porous and do not adversely affect the rate ofdeposition as the deposit becomes relatively thick (e.g., from 1 to 3centimeters or more).

In another embodiment of the invention, the porous silica preform isformed with layers of different composition. A quartz glass receptacle,for example, can be produced with an ultrapure inner layer. If thesilica preform for such a receptacle is formed of three layers and theouter layers have a coefficient of expansion less than that of themiddle layer, then after sintering, the outer layers of the glass willbe under compression. This makes possible the production of temperedglass with exceptional durability.

The electrophoretic deposition process of the present invention isparticularly well suited to the production of multi-layer silicapreforms of substantial thickness. An electrically-conductive shapingmold (positive anode) can easily be immersed in several differentslurries to cause the deposit of layers of any desired thickness. Thispermits the economical manufacture of special engineered silica glassproducts with improved properties and may make it possible to takeadvantage of the unrealized potential of glass or to eliminate or reducesome of the flaws which limit the strength and utility of most glassproducts.

Slip Casting

Another embodiment of the present invention relates to the use ofspecial porous silica molds for slip casting of quartz glass products.Such molds have unusual and unexpected advantages.

It has been discovered, for example, that there is a remarkable andunexpected reduction in the time required to slip cast a porous silicapreform when using a special high-porosity silica mold rather than aconventional plaster of Paris mold.

If a silica preform is produced from a suitable slip containingmicronized particles of high-purity silica by the gel-casting process ofthe present invention, it is possible to provide the preform with a highporosity of 20 to 30 volume percent and a highly uniform network of openpores of small size (e.g., a pore size of from 1 to 4 microns). It hasbeen discovered that this special internal open-pore structure isextremely well suited for slip casting, if the porous silica preform isused as a forming mold, and that the capillary attraction is superior tothat obtained when using conventional plaster of Paris molds. Thecapillary action in silica molds is apparently enhanced by thehydrophilic internal surfaces and also by microcracks in the mold.

Unlike plaster molds, a porous silica mold can be heated to atemperature of 1000° C. or more to provide the desired strength and canbe treated at high temperatures to modify or improve the mold structureor the character of the internal surfaces or to remove impurities.Unlike plaster molds, which are usually dried for two days or more at atemperature below 50° C., the silica mold can be dried in 30 to 40minutes or so at a temperature above 250° C. Like the m conventionalplaster molds, the silica molds are economical and reliable and can bemade at reasonable cost in a variety of sizes and shapes.

It has also been discovered that the bubble content of transparenthelium-sintered quartz glass can be greatly reduced by using silicamolds instead of the typical plaster molds to slip cast the preform.

When using silica molds in accordance with this invention, a novelsystem can be used to improve efficiency and to assure that the quartzglass products produced have the desired high quality. The preferredprocedure is to dry the silica mold after each slip casting operationhas been completed and after the slip-cast preform has been separatedfrom the mold. The typical cup-shaped open-top silica mold is turnedupside down, placed on a flat horizontal surface, and heated to atemperature of 250° to 400° C. or higher in such a manner as to create atemperature gradient and to cause radial outward flow of gases throughthe open-pore network, thereby removing unwanted impurities or causingthem to move away from the inner surface of the mold. Relativelyinexpensive heating means can be supported above the silica-mold to heatits outer surface to the desired temperature (e.g., 250° to 300° C.).

After the silica mold is dried, it is then ready for the next slipcasting operation. The dried silica mold provides a remarkably strongcapillary attraction and can greatly reduce the time required to producea typical silica preform (e.g., with a wall thickness of 5 to 8millimeters).

DEFINITIONS AND TERMINOLOGY

This invention is concerned with fused quartz and quartz glasscontaining a high percentage of silica as can be produced from quartzsand or from synthetic quartz of even higher purity. Quartz glassescommonly contain 99.5 to 99.99 percent by weight or more of silica andrarely contain more than one percent by weight of other compounds. Theterm “quartz”, as applied to glass, excludes high-silica glasses, suchas Vycor, containing 96 percent by weight of silica.

The term “high-purity quartz” as used herein refers to fused silica orquartz glass containing more than 99.99 percent by weight of silica andno more than about 50 parts per million (ppm) of contaminating metallicions. The term “ultra-pure” as applied to silica or synthetic quartzglass suggests a silica content of at least about 99.998 percent byweight.

The term “refractory” as used herein with respect to a glass or glasscomposition indicates the ability of the glass to withstand temperaturesas high as 1500° C. as are encountered in the casting of iron.

The dictionary term “micronized” is used herein with respect toparticles which have been ground or pulverized to provide an averageparticle size no greater than 20 microns.

The term “beta OH value” is used in its normal sense to indicate thehydroxyl content of a quartz glass as measured by infrared spectroscopy.Such term is defined and described in some detail in U.S. Pat. No.4,072,489. A conventional infrared spectrophotometer is used to measurethe transmissivities of the silicon hydroxyl vibrational bands in thenear infrared at wave lengths of a few microns. The beta OH value isbased on the transmittance of the sample at 2.6 microns and on thetransmittance at the OH absorption peak, which is about 2.73 microns forquartz glass. Such value is calculated using a logarithmic formula asset forth in U.S. Pat. No. 3,531,306. An infrared absorption beta OHvalue of 0.04 would indicate a hydroxyl content of less than 50 ppm.

The term “sintering temperature” is used herein to indicate atemperature of at least 1300° C. sufficient to cause the silica of theporous preform to coalesce, to cause the pores of the preform to close,and to obtain a high density.

Unless the context suggests otherwise, the term “vacuum” is used hereinto describe a substantial vacuum (i.e., a pressure of no more than 10torrs). A “high vacuum” is a subatmospheric pressure no greater thanabout 1 torr (1000 microns).

The term “reactive groups” as used herein with respect to a poroussilica body or silica particles refers to hydroxyl- orhalogen-containing groups (e.g., surface ≡Si—OH groups) or otherreactive or unstable groups which promote nitridation of the silica andchemical bonding of nitrogen atoms or amine groups to some of thesilicon atoms when the silica is heated in a suitable nitrogen reducingatmosphere.

As used herein, the term “colloidal silica” refers to extremely smallsilica particles having an average particle size of from 1 to 100namometers (i.e., less than 0.1 micron).

The term “ultrapure” as applied to natural or synthetic silica indicatesthat the metallic impurities other than aluminum do not exceed 4 partsper million (ppm). An “extremely pure” silica contains up to 15 ppm ofaluminum and a total of no more than 8 ppm of other metallic ions.

The term “high-porosity” as applied to a silica preform relates to thetotal volume of the pores or internal cavities rather than the width ordiameter of the pores and suggests a porosity of from 25 to 30 volumepercent or greater.

The term “transparent” as applied to sintered quartz glass is used inthe normal sense to define clear full-density glass and does not requireglass approaching optical grade. The term covers typical helium-sinteredglass containing a large number of bubbles with a diameter of from 1 to10 microns and a substantial number of larger bubbles including somethat are readily visible. Bubbles are considered “visible” if they havea diameter of at least 120 microns and are visible to the naked eyewithout magnification. A sintered transparent quartz glass is consideredto have sound optical quality and utility for some optical uses if ithas a minimal bubble content and no more than a small number of readilyvisible bubbles (i.e., an average less than one per square inch). In asintered glass of “minimal” bubble content, the average bubble size isless than 6 microns and the glass is usually substantially free of largevoids or bubbles with a diameter of 200 microns or more.

Silica particles are “electrophoretically mobile” in an aqueoussuspension or slip when the negative charge on the particles is suchthat they can be readily attracted to a positive electrode.

It will be understood that, unless the context suggests otherwise, partsand percentages are by weight rather than by volume.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is particularly concerned with the manufacture ofhigh-purity vitreous silica or quartz glass products from refractorysilica compositions which are molded and sintered to densify the glass.The methods and apparatus used in the practice of the invention may besimilar to those described in U.S. Pat. Nos. 4,072,489 and 5,053,359.

The invention involves formation of a shaped porous silica body orpreform from a refractory composition or a slurry of fine silicaparticles by slip casting (See U.S. Pat. No. 4,072,489), by hotisostatic pressing, by electrophoretic deposition, by injection molding(See U.S. Pat. No. 3,222,435) or by other suitable methods (See U.S.Pat. No. 3,619,440). A unique gel-casting process can also be employed.

High-purity silica or fused quartz may be pulverized, micronized orreduced to appropriate micron size as described in U.S. Pat. No.4,072,489 while maintaining the desired purity. A slurry or slipcontaining micronized particles of high-purity silica is preferablyprepared by wet milling in a conventional ball mill having balls orstones formed of essentially pure fused quartz. The liquid employedduring milling is preferably distilled water rather than an organicliquid. After milling, the average particle size of the silica particlesis from 2 to 10 microns.

In the practice of the invention, it is usually preferable to employhigh-purity silica or fused quartz with a silica content of 99.99percent or higher. Natural or synthetic fused quartz with a reportedsilica content of at least 99.999 percent by weight is availablecommercially. Such ultra-pure silica can be produced by hydrolysis ofsilicon tetrachloride or ethyl silicate (TEOS). Extremely pure silicacan also be produced from high-purity quartz sand that has been treatedto remove impurities.

Certain embodiments of the present invention involve the use of asol-gel process to form synthetic vitreous quartz of high purity from asilicon compound, such as silicon tetrachloride or a silicon alkoxide. Asol prepared from the silicon compound may be hydrolyzed to form asolution which is thereafter polymerized to form a silica gel.

The silicon compound is preferably an organo-silicate or alkoxide, suchas methyltrimethyloxysilane, tetraethyl orthosilicate (TEOS),tetramethyl orthosilicate (TMOS) or other alkyl silicate (e.g., propylsilicate). The organic compounds preferred for use in the practice ofthe invention are disclosed in U.S. Pat. No. 4,789,389 and have theformula Si(OR)₄ or Si R(OR)₃ where R is an alkyl group.

Ethyl silicate, for example, can be synthesized from silicontetrachloride and anhydrous ethyl alcohol. Partially hydrolyzed ethylsilicate (TEOS) is available commercially (e.g., SILBOND, a product ofAkzo Chemicals, Inc.), and can be used.

A silicon alkoxide, such as ethyl silicate, can be hydrolyzed in thepresence of an acid or base. The reaction can be described by a simpleformula where 1 mole of ethyl silicate plus 2 moles of watertheoretically produce 1 mole of silicon dioxide and 4 moles of ethanol,but actual hydrolysis is not this simple. Many intermediate species ofpolysilicates are formed during hydrolysis which grow in chain lengthuntil most or all of the ethyl groups are driven off and a non-linearnetwork of —Si—O—Si— remains. After the hydrolyzed ethyl silicate isgelled and dried, it forms silica.

Commercial ethyl silicate binders can be produced by partiallyhydrolyzing TEOS under carefully controlled conditions to provide astable mixture of polysilicon-oxygen “prepolymers” which can be storedand subsequently hydrolyzed to completion by adding an appropriateamount of water and changing the pH to an unstable range by using agelling agent.

The preparation of silica glass from silicon alkoxides is described inU.S. Pat. Nos. 4,622,056 and 4,789,389. The catalyst used for hydrolysismay be a base, such as ammonium hydroxide, or an acid, such ashydrochloric acid, nitric acid or formic acid. The gelling catalyst usedto adjust the pH of the hydrolyzed alkyl silicate may be an acid or abase. Excellent results can be obtained using a weak base, such asmorpholine or urea, to effect gelling. Other bases which may be usefulare disclosed in U.S. Pat. No. 4,680,048 (e.g., triethylamine, pyridineand aniline).

Prior Disclosure

Copending patent application Ser. No. 07/767,691, filed Sep. 30, 1991,describes an invention of Ted A. Loxley relating to a unique gel-castingprocess and to a process for producing transparent quartz glass ofexceptional quality using hydrolyzed organo-silicates or siliconalkoxides, such as ethyl silicate (TEOS), methyl silicate (TMOS)methyltrimethyloxysilane, or other organic silicon compound. Saidorganic compound, which is disclosed in U.S. Pat. No. 4,789,389, has theformula Si(OR)₄ or SiR(OR)₃ where R is an alkyl group.

The disclosure of the invention in said copending application Ser. No.767,691 is incorporated herein by reference and made a part hereof. Thepresent application is a continuation-in-part of that application.

That invention is particularly useful in the manufacture of transparenthomogeneous bubble-free glass of very high quality. For example,cristobalite-nucleated vitreous quartz glass receptacles or cruciblesmade in accordance with the invention of U.S. Pat. No. 5,053,359 or saidcopending application are greatly improved when the porous silica body(preform) is completely impregnated with a hydrolyzed silicon compoundor silica sol prior to sintering of the glass to full density. Suchsilica sol may be prepared by hydrolysis of silicon tetrachloride or aderivative thereof such as TEOS.

As disclosed in said patent application, the hydrolyzed TEOS fills thepores of the preform and functions to eliminate gas bubbles in the glassresulting from large pores in the preform. It also serves like asintering aid and permits sintering of the silica at lower temperatures.Optionally the TEOS in the pores of the silica preform can be gelled inan atmosphere of ammonia at room temperature, thereby assuringuniformity in the sintered product.

Impregnation of the pores of the porous silica preform with a hydrolyzedsilicon compound, such as TEOS, serves another purpose. For some reason,perhaps due to the formation of silicon monoxide gas, the TEOSfacilitates the removal of alkali metal ions. Near the outer surface ofthe glass product the percentage of alkali metal ions can be greatlyreduced, perhaps more than 50 percent in some cases.

In the manufacture of a slip-cast quartz glass receptacle, such as acrucible, the preferred procedure is to impregnate the entire silicapreform with a hydrolyzed silicon compound, such as TEOS, gel thesolution in ammonia, dry the preform, sinter the preform in a vacuumfurnace at a temperature of from 1200° C. to 1400° C. to a high density,and thereafter heat the densified preform in a vacuum furnace or in ahelium atmosphere to provide a transparent full-density quartz glass.

The aforesaid Loxley invention also includes a unique process for gelcasting which has great potential in the industry and makes itcommercially practical to produce high-quality silica products whichcould not be made by slip casting. The process involves the addition ofTEOS or other suitable organo-silicate to an aqueous slurry having ahigh silica content, such as 80 to 85 percent, and hydrolyzing the TEOSin situ during subsequent mixing. A gelling agent or catalyst, such asmorpholine or other weak base, is added to the hydrolyzed slurry justbefore casting to cause gelling or polymerization of the solution.

An illustrative example of the gel-casting process which follows isdescribed in said copending application. An aqueous slurry is preparedin a ball mill having a solids content of about 82 percent and a pH ofabout 2 to 4 and containing fused quartz particles with an averageparticle size of from 6 to 8 microns. A minute amount of micronizedparticles of basic aluminum acetate is thoroughly dispersed in theslurry and TEOS is then mixed with the slurry along with a small amountof hydrochloric acid to promote hydrolysis and ethanol to improvemiscibility. No additional water is added to the slurry. The liquid inthe original slurry provides the water for hydrolysis of the TEOS andmay be somewhat below or in excess of the stoichiometrically requiredamount. The mole ratio of water to TEOS can be 2:1 to 4:1 or higher.

The unique gel-casting method of this example helps to reduce some ofthe cracking and shrinkage problems associated with prior gel-castingprocesses, probably because the water responsible for the shrinkageproblem has been minimized in amount. This unique method is particularlywell suited to commercial manufacture of ultra-pure silica glassproducts and products that are unsuitable for conventional slip casting,particularly those having thick walls or complex shapes.

The basic Loxley invention is an important advance in the field ofquartz glass and may be defined broadly in a generic manner as follows:A gel-casting process for manufacture of shaped quartz glass productswherein an acidic slurry is prepared comprising from about 15 to about20 percent by weight of water and from about 80 to about 85 percent byweight of micronized silica particles, said slurry is fed into a shapingmold to form a silica body which is then dried and fired outside of themold to provide a shaped silica preform with a porosity of at leastabout 15 volume percent, and said preform is heated to a sinteringtemperature in an inert atmosphere or in a vacuum to cause the silica tocoalesce and to form a high-density quartz glass, said process beingcharacterized in that a liquid organic silicon compound is added to saidslurry, said compound having the formula Si(OR)₄ or SiR(OR)₃ where R isan organic hydrocarbon group; in that the mole ratio of said siliconcompound to the water present in said slurry is preferably from about1:6 to about 1:2; in that said silicon compound is hydrolyzed in situunder acidic conditions while being thoroughly mixed with the micronizedsilica particles; and in that a gelling agent is added to and mixed withthe acidic slurry to initiate polymerization before the slurry entersthe mold cavity.

It will be apparent that the above gel-casting process can be modifiedand improved in various ways. Refinements, modifications, embellishmentsand illustrative examples are described hereinafter to facilitate a morecomplete understanding of the invention.

The gel-casting process can be used to make various quartz glassproducts including flat plates as indicated hereinafter in Example VI.The firing of the silica body and the final sintering as described inthat example can be modified. For example the gel-cast silica body canbe dried, fired in a vacuum furnace for 5 to 8 hours or more at atemperature of 1100° C., and then sintered rapidly in helium in aninduction furnace to a temperature of about 1750° C. as in U.S. Pat. No.4,072,489 to produce a full-density transparent quartz glass.

The aqueous slurry employed for gel casting may also be modified byreplacing part or all of the high-purity vitreous silica particles withparticles of high-purity alpha-quartz (sand) or cristobalite. Themicronized silica particles of the slurry can, for example, consist offrom 25 to 75 percent by weight of vitreous silica and from 75 to 25percent by weight of crystalline silica. The final sintering in heliumto a temperature of 1750° C. above the melting point of cristobalite canproduce a transparent vitreous quartz product.

The gel casting process of the present invention is well suited to themanufacture of quartz glass products which are large or relatively thickand not well suited to manufacture by arc-fusion or slip-castingmethods. When making thick parts it may be desirable to employ a slurrywith particles of greater average particle size than normally used(e.g., 8 to 15 microns) or to add to the slurry a minor amount of largerparticles.

The sintered quartz glass products made by the gel-casting orimpregnation processes and techniques described above and in saidcopending application Ser. No. 767,691 are unique in that a minorportion of the silica in the porous, silica preform is ultra-puresynthetic silica derived from a silicon alkoxide.

These inventions make possible the manufacture of ultra-pure transparentvitreous silica products of extremely high quality which have adequatehomogeneity for some optical uses or for possible use in opticalwaveguides.

Heretofore optical glass of very high quality has been made fromultra-pure synthetic silica produced, for example by flame hydrolysis ofsilicon tetrachloride (See Corning Patent No. 3,806,570). One commercialsynthetic optical glass (Corning Code 7940) manufactured by flamehydrolysis has excellent optical qualities and exceptional transmittancein the ultra-violet. The refractive index, birefringence constant andhigh transmittance of this premium-quality synthetic quartz glass makesit well suited to many optical uses.

The present invention makes possible manufacture of quartz glass withthe clarity and transmission characteristics needed for some opticaluses. If desired, a hipping furnace may be employed to assure removal ofany bubbles or voids remaining after sintering.

Nitrided Quartz Glass

A preferred embodiment of the present invention relates to themanufacture of high-purity nitrided silica glass products. In thisembodiment a silica body or preform with a substantial porosity isformed or molded to the desired shape, dried, fired in air or oxygen,and then nitrided in a nitrogen containing reducing atmosphere to causebonding of nitrogen atoms to silicon atoms of the silica (e.g.,≡Si—NH₂). The nitridation of the silica preform can be carried out priorto or during sintering of the preform.

The silica of the preform is preferably a vitreous quartz of high purityand may have a purity in excess of 99.99 percent by weight. If thesilica preform is formed by gel casting as in the embodiment of theinvention-previously described, a substantial portion of the silica willbe formed from hydrolyzed ethyl silicate. Before the silica preform issintered to high density, it can be fired for 2 to 10 hours in air at atemperature of from 800° C. to 1250° C. to oxidize the hydrocarbons orother combustibles. A normal firing might be for about 3 to about 4hours at a temperature of from 1050° C. to 1200° C.

When making nitrided quartz glass products according to this invention,the porous silica preform is provided with surface reactive groups, suchas hydroxyl or halogen groups, which are unstable in the presence ofammonia and promote the nitridation of the silica at high temperatures.The amount of the hydroxyl groups or other reactive groups can besubstantial, (i.e., at least 100 parts per million, ppm). That amountcan be from 150 to 250 ppm or more.

In order to provide the desired hydroxyl content, the porous silicapreform can be hydroxylated with steam at a high temperature, such as400° C. to 1100° C. or more. The steam treatment increases the hydroxylcontent at least about 30 percent and preferably at least about 50percent. The treatment can be carried out before or during the firing ofthe preform in air or oxygen by having an appropriate amount of steampresent during oxidation of the combustibles. If oxidation is effectedin an atmosphere of oxygen and steam, the firing temperature could be aslow as 500° C. The conditions during hydroxylation of the silica preformshould be selected to obtain the desired hydroxyl content prior to thenitriding step.

When the hydroxylated silica preform is nitrided under suitableconditions in a nitrogen-containing atmosphere, there is a reduction inthe hydroxyl content and a commensurate increase in the amount ofchemically-bound nitrogen. That amount should be an effective amount, noless than 25 ppm, and is preferably at least about 50 ppm. Thenitridation can be carried out in such manner that the hydroxyl contentis reduced 50 to 70 percent or more. The nitriding step can reduce thehydroxyl content of the quartz glass product to a low value, such as 10to 20 ppm.

A dramatic improvement in the physical properties of high-puritysintered quartz glass can be achieved with a minute amount ofchemically-bound nitrogen (e.g., less than 0.01 percent by weight). Theminimum amount depends on the intended use for the quartz glass product.

When the desired minimum nitrogen content in the glass product is 100ppm or more, an adequate amount of hydroxyl groups or other reactivegroups should be provided in the porous silica preform. For example,hydroxylation of the preform may be desirable to provide an hydroxylcontent of 150 to 200 ppm or more prior to nitriding. After nitridingthe preform and sintering to almost full density, the infrared beta OHvalue may be very low.

Nitridation of the porous silica preform is more effective when using astrong reducing agent, such as anhydrous ammonia, to provide ahydrogen-nitrogen reducing atmosphere. It is also possible to use otherreducing agents, such as hydrazine. An atmosphere suitable for effectivenitridation can be obtained from cracked ammonia. A reducing atmosphereconsisting of a corresponding mixture of nitrogen gas and hydrogen gasis less effective and may require more time to achieve the desiredresult. In carrying out the process of the present invention, thenitriding in the hydrogen-nitrogen reducing atmosphere is carried out ata temperature of from 600° C. to 1300° C., preferably from about 900° C.to about 1100° C.

The partial replacement of surface hydroxyl groups With chlorine by apretreatment step may enhance the nitriding reaction.

Useful nitrided quartz glass products can be made according to thisinvention when using various nitrogen-containing reducing atmospheresduring the nitriding step. If the reducing atmosphere is obtained fromcracked ammonia, nitriding can, for example, be carried out for 30minutes to an hour or more at a temperature of from about 900° C. toabout 1100° C. If the reducing atmosphere consists of a mixture ofnitrogen gas and a reducing gas, such as hydrogen, methane or carbonmonoxide, nitriding could be carried out for 1 to 2 hours or more at atemperature of from 1000° C. to 1200° C. or more.

After the silica preform has been nitrided to provide the desirednitrogen-silicon bonding and the desired content of chemically-boundnitrogen, it is sintered to a high density, such as 98 to 99 percent.The final sintering may be effected in a conventional electric inductionfurnace at high temperatures, such as 1550° C. to 1750° C. Since thesilicon-nitrogen bonds are unstable at temperatures above 1500° C., itis desirable to carry out the final sintering operation in a nitrogenreducing atmosphere. However, if the sintering is carried out in twostages and the pores of the preform are closed (See Example VI), thefinal sintering can be carried out in helium or argon or other inertgas.

Nitrided quartz glass products made according to the present inventionhave remarkable physical properties even when the amount ofchemically-bound nitrogen in the glass is very small. For example, theviscosity of quartz glass at 1260° C. can often be increased 50 percentor more by a nitriding treatment which provides the glass with anitrogen content of less than 0.02 percent by weight. The same nitridingtreatment may at the same time provide the quartz glass with anincredible increase in the resistance to devitrification.

A proper basis for comparison can be provided by slip casting twoidentical silica preforms from the same slurry (e.g., the slurry ofExample I). One porous preform is dried, fired in air for about 3 hoursat 1150° C. to oxidize the combustibles, and then sintered in helium ina semi-automatic induction furnace as described in U.S. Pat. No.4,072,489. If the other identical preform is dried and fired inessentially the same way but subjected to a nitriding treatment inammonia at 1100° C. (as in Example I) and then sintered in nitrogen toalmost full density in the same induction furnace in essentially thesame way, the improvement in the physical properties can be almostunbelievable.

While superior results can be obtained by sintering in two stages or byusing two or more furnaces as advocated in the illustrative exampleswhich follow, it will be understood that some advantages of theinvention can be obtained using a single electric induction furnace ofthe type disclosed in U.S. Pat. No. 4,072,489 in which nitriding iseffected in a reducing atmosphere consisting of nitrogen gas and areducing gas, such as hydrogen, methane or carbon monoxide, and at arelatively high temperature, such as 1400° C. to 1600° C. or higher, ina short period of time, such as 10 to 30 minutes. After nitriding thesintered preform may be further heated to a temperature above 1700° C.to provide a high density, such as 98 to 99 percent by weight, and/or toeliminate cristobalite.

While the porous preform is nitrided and sintered in the inductionfurnace at a temperature above 1400° C., it is preferable to causepressurized nitrogen gas to flow through the preform as by use of aperforated or porous graphite support, such as the graphite mandrel (16)described hereinafter in Example I. The porous graphite support usedduring sintering of a glass crucible could, of course, be concave orconvex.

When making simple articles, such as quartz glass receptacles, slipcasting is usually employed to form the porous preform prior tonitriding. When conventional plaster of Paris molds are used for slipcasting, the calcium ions introduced into the preform serve as acatalyst to promote nitriding. It will be understood, however, that thepresence of calcium ions is not essential and that the preform need notbe formed by slip casting.

Electrophoretic Deposition

Another important embodiment of the invention relates to the manufactureof quartz glass articles by an electrophoretic deposition process asdescribed heretofore and as illustrated hereinafter in Example VI.

The process is preferably carried out using an aqueous slurry consistingof pure water and high-purity micronized silica particles which havebeen provided with a negative electrical charge sufficient to provideelectrophoretic mobility. Such a high-purity slurry can be prepared bywet milling silica in a ball mill for 24 to 36 hours and preferably hasa solids content of at least 80 percent by weight. The micronized silicaparticles usually have an average particle size of from 5 to 10 micronsand usually consist of fused quartz but can include substantial amountsof crystalline silica (e.g., cristobalite or alpha quartz). It issometimes advantageous to employ a slurry wherein 25 to 75 percent byweight of the silica is crystalline silica.

The particle size distribution in the slurry can be important and theparticle sizes are preferably preselected or kept within predeterminedranges to assure that the deposits at the positive anode are porous anddo not seriously interfere with formation of a relatively thick deposit.The silica particles of the slurry should be substantially free ofcolloidal silica and preferably comprise no more than one percent byweight of silica with a particle size of one micron or less and no morethan 40 percent by weight of silica with a particle size of from 2 to 3microns. The silica particles usually consist of at least 40 percent byweight of particles with a particle size of from 6 to 20 microns.

The process of this invention may be carried out using relatively simpleequipment including one or more tanks or reservoirs with a volume of 20to 40 gallons, each having a rotary mixer or the like, one or morenegative anodes, and electrical means for providing a direct current tocause electrophoretic deposition on positive anodes immersed in theslurry. The slurry containing the micronized silica particles preferablyhas a pH of from about 3 to 5 and usually contains added ammonium ionsthat improve the conductivity of the slurry without contaminating thesame with unwanted metallic ions.

In the manufacture of receptacles, such as crucibles, acid tanks, belljars and the like, a number of cup-shaped metal shaping molds can beused which serve as positive anodes with electrically conductive shapingsurfaces on which the silica is deposited. In a typical cup-shapedaluminum mold with a diameter of 4 to 12 inches and a height of 2 to 10inches, for example, a relatively thick deposit can be formed byelectrophoresis in a relatively short period of time.

As the silica is deposited at the mold (anode), the percentage of waterin the vicinity of the anode tends to increase and the average solidscontent of the slurry will gradually decrease. The process of thisinvention is preferably carried out in such manner that the solidscontent of the slurry near the anode is always maintained at 80 percentby weight or higher. This can be achieved by agitating the slurry in thetank to cause excess water to move upwardly and by periodically addingsilica particles to the slurry or by removing water from the slurry. Ifdesired a permeable membrane can be used to separate water from theslurry. The agitation of the slurry in the tank or reservoir ispreferably effected by a conventional rotary mixing means and ispreferably continuous to maintain a homogeneous mixture.

Electrophoretic deposition by the process of this invention facilitatesthe manufacture of special engineered quartz glass products which cannotreadily be produced by other methods. A layer of any desired thicknesscan be deposited at the anode or shaping surface by temporarilyimmersing the anode in the slurry for a predetermined period of time,and a multilayer silica body can readily be produced by immersing thesame anode in a series of different slurries in different tanks. Byproviding a series of different tanks, each containing 10 or moregallons of a suitable slurry and the associated means for carrying outelectrophoretic deposition, a porous multilayer silica body or preformcan readily be formed by immersing the shaping mold or anode in severaldifferent slurries sequentially.

In this manner it is convenient to form a porous silica preform havingan ultrapure inner layer and other layers of lesser purity or a preformhaving a middle layer with a coefficient of expansion greater than thatof outer layers, whereby the article produced after the final sinteringin an induction furnace is a tempered quartz glass with outer surfaceportions under compression. A higher coefficient of expansion in themiddle layer can be obtained, for example, by changing the chemicalcomposition of the silica particles in the slurry (e.g., by adding analuminum compound, such as aluminum oxide).

Various engineered glass articles can be produced from specialmultilayer silica preforms made by electrophoretic deposition. Itbecomes possible to provide unique products and to realize the untappedpotential of glass, perhaps by reducing the microflaws which are presentin all glass products and limit the strength and durability of theglass.

Multilayer silica preforms make possible the manufacture of unique andunusual quartz glass products with special characteristics. For example,a cup-shaped silica preform can be produced with an ultrapure innerlayer and a number of additional layers with different calcium contentsthat increase in each successive layer. When such a preform is nitridedin accordance with the first-described embodiment of the presentinvention, the calcium acts as a catalyst, and the degree of nitridingcan be increased accordingly so as to be greatest at the outer surfaceportions.

The electrophoretic deposition process of this invention and theequipment used to carry out the process as described above can also beused in a somewhat different manner for rapid production of thick quartzglass products by employing relatively large pieces of fused quartz tofill an electrically conductive shaping mold (anode) and then depositingmicronized silica particles in the void spaces between such pieces. Thepieces may, for example, have a width or diameter of from 0.3 to 0.8inch or more.

Crystalline Silica Preforms

The processes disclosed in the present application are well suited tothe manufacture of extremely pure transparent or full density quartzglass products and involve the use of porous silica preforms preferablymade from aqueous slurries of exceptional purity. Ultrapure quartz glasscan be very expensive, especially when made from fused silica particleshaving a purity of 99.995 to 99.999 or better. On the other hand,treated quartz sand of extremely high purity is much less expensive.

Substantial savings and other benefits can be achieved when part oralmost all of the vitreous (fused) quartz in the refractory compositionis replaced by alpha quartz. It is much easier to remove impurititesfrom quartz sand than to produce extremely pure fused quartz. A largeamount of electrical energy is required for the electric furnaces usedto produce fused quartz. The savings in electrical energy that can beachieved by use of alpha quartz in the refractory compositions used aresignificant and perhaps as much as 3 kilowatt hours per pound.

The cost of producing extremely pure quartz glass can, for example, bereduced by providing a slip or slurry made from a refractory compositionconsisting essentially of no more than 75 percent by weight of vitreoussilica and from 25 to 75 percent by weight of alpha quartz with a purityof from 99.99 to 99.999 percent or greater.

For example, in the manufacture of ultra-pure quartz glass products inaccordance with this invention, the silica particles of the refractorycomposition can be wet milled in a ball mill to an average particle sizeof from 2 to 10 microns and can consist of 30 percent by weight ofhigh-purity fused quartz particles and 70 percent by weight ofhigh-purity alpha quartz (or cristobalite) particles. The crystallinequartz would be milled separately and then mixed with the micronizedfused quartz particles. The slip-casting procedure and subsequentsintering operations could be essentially the same as when using fusedsilica particles only.

The final heating would preferably be to a temperature of about 1750° C.to form a full-density or transparent vitreous quartz productessentially free of cristobalite.

The use of substantial amounts of crystalline silica in the slurry canprovide important advantages. The crystalline silica can slow down thesintering to permit longer sintering times or to assist in retaining theopen-pore structure. The use of alpha quartz or cristobalite particlesin the slurry makes it possible to employ a special high-temperaturechlorine treatment process to remove impuritites from a porous silicapreform.

For example, a silica preform containing 40 to 60 percent or more ofcristobalite or alpha quartz with a purity of about 99.99 percent couldbe treated with chlorine gas one or more times at a high temperature offrom 1100° C. to 1250° C. to remove contaminating metallic ions andsubstantially increase the purity of the silica preform.

In order to provide an effective chlorine treatment the porous silicapreform is alternately subjected to a vacuum and to chlorine gas so thatall of the gases are removed from the pores and the pores are thenfilled with chlorine gas while the preform is heated for 30 minutes ormore. This cycle can be repeated one or more times to achieve furtherimprovement in the purity of the silica body or preform.

The methods and procedures described and the unique refractorycompositions advocated in the examples of this specification or setforth as preferred embodiments are merely illustrative and are notintended to limit the scope of the invention. They are practicable andshould be appropriate and useful in attaining the major or importantadvantages of the invention.

EXAMPLE I

A high-purity fused quartz slurry is prepared by wet milling high-purityfused quartz (99.99% SiO₂) using ultra-pure deionized water and ahigh-purity fused quartz grinding media in a conventional ball mill. Thesilica particles are milled for about 24 to 36 hours to provide a slipor slurry with a pH of from 2 to 4, an average particle size of fromabout 5 to about 7 microns, and a solids content of from about 82 to 84percent by weight. The pH of the slurry is adjusted to about 7.5 byadding some dilute ammonium hydroxide, and the slurry is mixed for anextended period of time to break up floccules or agglomerates.

In the making of a quartz glass receptacle in accordance with thisinvention, the above slurry can be employed for slip casting acup-shaped silica body or preform in a conventional plaster of Parismold using conventional drain-casting procedures similar to thosedescribed in U.S. Pat. Nos. 3,972,704 and 4,072,489. The slip-castpreform is air dried for about 24 hours in a warm drying room maintainedat a temperature somewhat above 40° C. and then placed in an electricfurnace and gradually heated to about 800° C. in an atmosphere of airand superheated steam. The preform is maintained at about 800° C. insuch atmosphere for about 2 hours and then heated to about 1200° C. insuch atmosphere for an additional 2 hours to assure complete oxidationof the hydrocarbons or combustibles. The amount of vaporized water orsteam is adequate to provide the desired vapor pressure and to maintainthe desired hydroxyl content in the preform.

After such firing the preform has a porosity of from 15 to 20 volumepercent and should have adequate strength for handling. Except for ahigh hydroxyl content, the porous silica preform is conventional. Thewall thickness is substantially uniform and may be from 5 to 7millimeters, for example, in a cup-shaped receptacle with a diameter of25 to 30 centimeters.

The preform is then placed in an electrically-heated vacuum furnace anda substantial vacuum is applied for about 15 minutes to remove air andsteam from the internal pores. The pressure is preferably reduced below2 torrs. The vacuum is then discontinued and anhydrous ammonia isadmitted to the furnace and caused to fill said pores. The preform isthen heated in the ammonia (nitrogen-hydrogen) reducing atmosphere forabout 30 to 35 minutes at a temperature of about 1100° C. to causenitridation at the inner surfaces of the pores.

Thereafter, the porous nitrided silica preform is cooled enough topermit handling and is mounted upside-down on the heated hollow graphitemandrel (16) of a semi-automatic electric induction furnace of the typedisclosed in said U.S. Pat. No. 4,072,489. The preform is shaped to fitthe mandrel. The sintering procedure may be similar to that disclosed inthat patent, but the glass is sintered in a nitrogen reducing atmosphererather than in helium. The mandrel is perforated and internallypressurized with nitrogen to cause the nitrogen to flow radiallyoutwardly through the mandrel and the porous preform. Just before thepreform enters the furnace chamber, the temperature of the mandrel andthe furnace may be around 1300° C. to 1500° C. The mandrel and theporous preform are advanced into the furnace chamber to start thesintering cycle, and the furnace is heated at a rapid rate to increasethe glass temperature from below 1400° C. to above 1600° C. andgradually to more than 1700° C. to cause the silica particles tocoalesce and the glass to reach a high density.

The glass is preferably heated to about 1750° C. to assure that nocristobalite remains in the glass. When the glass reaches thattemperature, as indicated by a pyrometer, the mandrel and the sinteredreceptacle are retracted out of the furnace and cooled to permit removalof the receptacle. The total time for the sintering operation in theinduction furnace can be relatively short to minimize cristobaliteformation and depends on the thickness of the glass and also the initialtemperature of the mandrel. It may take from 8 to 12 minutes to providethe desired sintering.

A non-oxidizing atmosphere of nitrogen gas is provided in the inductionfurnace during the sintering operation which tends to avoid undesireddecomposition reactions at the surfaces of the internal pores of thesilica body which may occur at high temperature due to the instabilityof the silicon-nitrogen bonds. Such bonds are more stable in thepresence of a nitrogen atmosphere or a nitrogen-hydrogen reducingatmosphere.

Sintering of the glass in nitrogen as described above produces a white,opaque quartz glass with a substantially uniform cellular structure. Thesintered quartz glass receptacle can have a density in excess of 98.5percent by weight and pores of minute size up to 4 microns.

The procedure described above can be modified in various ways. Theammonia atmosphere employed for nitridation of the preform can bereplaced by a similar reducing atmosphere consisting of a mixture ofnitrogen and hydrogen or carbon monoxide. A similar nitrogen-hydrogenmixture can also be used in the induction furnace during the finalsintering operation.

The quartz glass receptacle produced by the procedure of this Example Ihas remarkable properties which one would not expect in view of thesmall amount of chemically-combined nitrogen actually present in theglass structure. The nitriding of the quartz glass effects a greatincrease in the viscosity of the glass at high temperatures above 1400°C. and an incredible increase in the resistance of the glass todevitrification.

Although the glass has a density of 98.5 percent or more, it providesexceptional resistance to radiation heat transfer because of the uniformnetwork of minute pores. Such a glass has potentially great commercialvalue with respect to radiation heat shields, particularly for furnacesused in the semi-conductor industry.

EXAMPLE II

If a porous silica preform corresponding to the preform of Example I issintered in a helium atmosphere to produce a clear full density quartzglass receptacle, it may be advantageous to fill the pores of thepreform with a silica gel prior to firing. The preferred procedure is toimpregnate the entire silica preform with a tetraethyl orthosilicate(TEOS) which has been hydrolyzed with a suitable acid or base. Suchhydrolyzed TEOS can be produced from a mixture of TEOS (i.e., SILBONDPURE) with water and a small amount of hydrochloric acid as disclosed,for example, in U.S. Pat. No. 4,789,389. The mole ratio of water to TEOScan be from 6:1 to 10:1.

In this Example II a porous silica preform identical to the preform ofExample I is slip cast and dried for 24 hours at a temperature of about43° C. and then soaked and completely impregnated with the hydrolyzedTEOS. After draining and removing the excess TEOS from the preform, thepreform is placed in a closed container with ammonium hydroxide for 15minutes or more to gel the TEOS which substantially fills the pores ofthe preform. Thereafter, the preform is dried for about 24 hours in airat about 43° C. and then fired in air in an electric furnace for about 3hours at about 1100° C. to oxidize the combustibles.

The resulting porous silica preform with a density of about 88 percentis then removed from the furnace, placed on the perforated mandrel (16)of the semi-automatic induction furnace and sintered in the mannerdescribed in Example I to a temperature of 1750° C. The sintering can becarried out in a helium atmosphere (or in a vacuum) according to saidU.S. Pat. No. 4,072,489 and can produce a full density transparentquartz glass receptacle.

EXAMPLE III

A slip-cast silica preform impregnated with hydrolyzed TEOS as inExample II can be hydroxylated with superheated steam, nitrided withanhydrous ammonia, and sintered to high density in nitrogen generally inthe manner described in Example I to produce an opaque all-vitreousquartz glass receptacle comparable to the receptacle of Example I.

In this Example III, the dried slip-cast preform is impregnated withhydrolyzed TEOS and treated with ammonium hydroxide to gel the TEOS asdescribed in Example II. Thereafter the preform is dried in air at 43°C. for 24 hours and then placed in an electric furnace, heated to about800° C., maintained at that temperature for about 2 hours in anatmosphere of air and superheated steam to hydroxylate the silica, andfurther processed, nitrided and sintered to high density in an inductionfurnace following the specific procedures described in Example I.

EXAMPLE IV

A high-purity fused quartz slip or slurry can be prepared essentially asdescribed in Example I and has a pH of from 2 to 4, an average particlesize of from 5 to 6 microns, and a specific gravity of about 1.84(indicating a solids content of about 83.7 weight percent and a watercontent of about 30 volume percent).

Ten liters of this slurry are added to a cylindrical mixing vessel witha capacity of 30 to 40 liters formed of a suitable material, such aspolyethylene. The vessel is equipped with a rotary mixer having a4-blade mixing head with a diameter of about 15 centimeters which isnormally driven at a speed of from 100 to 120 revolutions per minute.

After the aforesaid amount of the slurry has been added to the mixingvessel, the rotary mixer can be started and 10 milliliters (ml) ofhydrochloric acid added gradually. The mixing at 100 to 120 rpm iscontinued while adding about 930 ml of TEOS (i.e., SILBOND PURE) andcontinues for one hour to allow hydrolysis to proceed. The reaction isexothermic and can cause the temperature of the slurry to rise above 50°C. (e.g., to perhaps 55° C. to 60° C. or higher). The rotary mixercontinues to operate while the slurry cools.

After a few hours, when the slurry is at room temperature or atemperature below 25° C. and while the mixing continues, a small amount(e.g., 50 ml) of a one percent solution of morpholine in water is addedvery slowly, preferably over a period of 5 minutes or more depending onthe amount used. The amount of morpholine used is selected to providethe desired gel rate and is such that the slurry remains flowable untilall the morpholine is added even if partial gelling does occur. Theslurry should be poured or caused to flow into the mold while it stillflows easily (The ethyl alcohol produced during hydrolysis of the TEOSserves to improve the flow characteristics even if substantial gellingdoes occur).

The slurry is poured into a mold designed to produce a flat glass platewith a thickness of about 20 mm and diameter of 20 to 25 cm. Soon afterthe mold cavity is filled, the hydrolyzed TEOS begins to gel. The moldedgel-cast preform should be allowed to gel in the mold for 1 to 2 hoursor more and preferably remains in the mold overnight (e.g., 8 hours ormore). By then it should have adequate strength and can be removed fromthe mold and dried in air at about 43° C. for another 24 hours.

The dried porous silica preform is then placed in a drying oven,gradually heated from room temperature to 120° C. at a slow rate(e.g.,less than 50 degrees C. per hour) over a period of 2 to 3 hours,and held at about 120° C. for 48 hours to evaporate the free water andalcohol. The dried silica preform has a substantial porosity.

The dried gel-cast preform is then placed in a resistance-heatedelectric furnace and gradually heated in air to a temperature of about1100° C. The rate of heating is low, preferably no more than 10 degreesC. per minute. The preform may be heated at 1100° C. for 1 to 2 hours ormore in air to oxidize all of the combustibles (A shorter time and alower temperature would be adequate if the furnace atmosphere wasessentially oxygen rather than air).

Optionally, if a low hydroxyl content is required, the porous silicapreform may be placed in a vacuum furnace after the above-describedoxidation of the combustibles.

The optional vacuum-drying operation involves heating the fired preformin a vacuum furnace at a temperature of from about 1170° C. to about1180° C. for about 3 hours to reduce the hydroxyl content of the glassto less than 50 parts per million (ppm) by weight. The subatmosphericpressure in the furnace during such heating is preferably less than 2torrs.

After the porous silica preform is fired as previously described toeliminate combustibles, it can be removed from the furnace and placed inanother resistance-heated electric furnace. The preform can be initiallysubjected to a substantial vacuum for a period of 10 to 15 minutes toremove gases from the internal pores and to reduce the pressure below 2torrs. The operation of the vacuum pump is then stopped and helium gasis caused to flow into the furnace chamber and to fill the pores of thepreform. The silica preform is heated gradually to a temperature ofabout 1400° C. or more over a substantial period of time, such as 1 to 2hours or more, and in such manner that the preform is partially sinteredto close the pores.

Thereafter the gel-cast silica preform with a density of about 95percent can be placed in an electric induction furnace of the generaltype disclosed in said U.S. Pat. No. 4,072,489 and heated for 10 to 15minutes in an inert atmosphere of helium or argon to a temperature ofabout 1750° C. to eliminate cristobalite and to sinter the quartz glassto full density. The final sintering in the induction furnace may besubstantially as described in said U.S. Pat. No. 4,072,489.

A sintered product made according to this Example IV is a flat clearquartz glass plate of high quality having a thickness of about 20millimeters. A similar procedure is suitable for making full-densityquartz glass plates, receptacles or other shaped articles with athickness of 20 to 30 millimeters or more.

The procedure advocated in this Example IV can produce clear quartzglass plate of good quality when using varying amounts of TEOS. Forexample, satisfactory results can be obtained when the amount of TEOS isincreased from 930 ml to 1240 ml or is decreased from 930 ml to 620 mlper liter of slip.

EXAMPLE V

A high-purity slip-cast silica preform prepared in the manner describedin Example I and having a porosity of about 15 volume percent isimpregnated with hydrolyzed TEOS as in Example III. The TEOS is gelled,and the preform is dried in air at about 43° C. for about 24 hours inthe manner described in Example III. The preform is then fired in air inan electric furnace at atmospheric pressure for 3 hours at about 1100°C. to oxidize and burn out the organic combustibles. The resultingcup-shaped silica preform can have a porosity of about 12 volumepercent.

The preform can then be placed upside down in an electric resistancefurnace, heated to about 1400° C., to 1500° C. and maintained at atemperature in that range for a period of time sufficient to increasethe density to about 95 percent and to close the pores of the quartzglass article. During such heating, a high vacuum should be maintainedin the furnace chamber, the pressure being less than 200 microns.

The resulting quartz glass receptacle can then be removed from thevacuum furnace, placed on the hot mandrel (16) of the semi-automaticinduction furnace, and sintered in an inert atmosphere to a temperatureof about 1750° C. substantially as described in U.S. Pat. No. 4,072,489and in Example I. This can produce a full-density transparent quartzglass receptacle that has good optical properties.

It will be understood that the hydrolyzed TEOS used to impregnate theporous silica preform of Examples II, III, and V may be replaced withanother hydrolyzed silicon alkoxide and that hydrolysis may be effectedin the presence of ammonium hydroxide or other base (e.g., a Lewis base)rather than hydrochloric acid or other acid.

EXAMPLE VI

A slurry is prepared by wet milling ultrapure fused quartz usingdeionized water and ultrapure fused quartz grinding media in a ballmill. The silica particles are milled for about 30 hours to provide anextremely pure slip or slurry with a pH of from 2 to 4, an averageparticle size of from about 6 to about 8 microns, and a solids contentof from about 82 to about 84 percent by weight. The milling time isadequate to provide negatively charged micronized silica particles withelectrophoretic mobility. Dilute ammonium hydroxide is slowly added tothe slurry which is thoroughly mixed to provide an extremely pureelectrically conductive slip with pH of from 4 to 5.

About 20 gallons of a slurry prepared in this manner and having a solidscontent of at least 80 percent by weight is placed in a large tank orreservoir containing a negative electrode and a rotary mixer. Acup-shaped electrically conductive metal shaping mold with a diameter ofabout six inches and a height of about three inches is immersed in theslurry to provide a positive anode and a direct electric current isimposed to attract the silica particles to the anode.

The voltage is regulated and can be about 100 to 120 volts while thethickness of the deposit on the shaping mold rapidly increases to 0.2 to0.3 inch or more. If desired the voltage can be increased up to 200volts to speed up the deposition after the deposit is relatively thickas when making large articles with a wall thickness of 0.4 to 0.5 inchor greater.

The slurry is continuously agitated by the rotary mixer during theelectrophoretic deposition process to maintain the silica particles insuspension. The rotary blades of the mixer preferably have a speed of atleast 150 revolutions per minute (rpm) high enough to control the watercontent of the slurry adjacent to the anode and to maintain substantialuniformity in the slurry.

After the cup-shaped porous silica body or preform deposited on theshaping mold has the desired wall thickness (e.g., a thickness of from0.25 to 0.3 inch), the mold is removed from the slurry, the silica bodyis removed from the mold, dried and then fired at a temperature of 1050°C. to 1100° C. The resulting product with a porosity of at least 20volume percent can then be sintered in helium for 8 to 10 minutes ormore in an induction furnace to a temperature of about 1750° C. toprovide a full density transparent glass receptacle. The final sinteringin the induction furnace may be substantially as described in theaforesaid U.S. Pat. No. 4,072,489.

It will be understood that a procedure of the type described in thisExample VI can be employed in the commercial manufacture of a variety ofquartz glass receptacles and other products. The tank or reservoir canbe of adequate size to accommodate at least several metal shaping molds(anodes) and more than one cathode, and additional slurry can be addedor excess water removed to maintain the desired solids content.

It will be understood that variations and modifications of thecompositions, methods, devices and products disclosed herein may be madewithout departing from the spirit of the invention.

What is claimed is:
 1. A process for making a shaped quartz glassarticle of high purity with high resistance to devitrification wherein arefractory composition consisting essentially of fine particles ofhigh-purity silica is shaped to form a porous silica preform with aporosity of from 10 to 40 volume percent, said process comprisingnitriding the preform for 30 minutes or more in ahydrogen-nitrogen-containing reducing atmosphere at a temperature of atleast about 850° C. to provide the preform with a substantial amount ofchemically-combined nitrogen no more than about 250 ppm, and thereafterheating the porous preform to a sintering temperature above 1700° C. tocoalesce the silica particles.
 2. A process according to claim 1 whereinthe nitriding of the porous preform is carried out in a reducingatmosphere obtained from cracked ammonia at a temperature of from about900° C. to about 1100° C.
 3. A process according to claim 1 wherein thenitrided preform is presintered for 1 to 3 hours at a temperature offrom 1400° C. to 1500° C. to increase the density above 90 percent andto close the pores before the final sintering to a temperature above1700° C.
 4. A process according to claim 1 wherein said silica preformis hydroxylated to increase the hydroxyl content at least 50 percent andis then nitrided in a nitrogen-hydrogen reducing atmosphere at atemperature of at least about 1000° C. to provide the glass article witha nitrogen content of at least about 100 ppm.
 5. A process for making asintered quartz glass article from a fired preform comprising a shapedporous silica body formed from a slurry consisting essentially of waterand high-purity silica particles having an average particle size of from2 to 10 microns wherein no more than 75 percent by weight of the silicaof said slurry is vitreous silica and from 25 to 75 percent is alphaquartz and wherein said preform is sintered at a temperature above 1720°C. to remove cristobalite, to coalesce the silica particles, and to formquartz glass.
 6. A process according to claim 5 wherein the micronizedsilica particles of said slurry comprise from 25 to 75 percent by weightof alpha quartz particles with an average particle size of from 2 to 8microns and a purity of at least 99.99 percent.
 7. A process accordingto claim 5 wherein the preform is nitrided in a nitrogen-containingreducing atmosphere at a temperature of at least about 1000° C. toprovide the quartz glass with a nitrogen content of at least about 50ppm.
 8. A process according to claim 5 wherein the porous silica preformcontains at least 40 percent by weight of alpha quartz and retains theopen-pore structure when calcined for at least 30 minutes at atemperature up to 1250° C.
 9. A process for making a quartz glassarticle from a fired preform according to claim 5 wherein said silicabody is a receptacle formed by slip casting, gel casting orelectrophoretic deposition.
 10. A process according to claim 5 whereinsaid silica body is a receptacle formed by slip casting, at least 40percent by weight of the silica of the slurry is alpha quartz, and thefired preform is sintered to form a transparent quartz glass.